This invention relates to a method of evaluating the profile of dopant concentration in the vicinity of the growth interface between a vapor phase growth layer and a substrate of silicon epitaxial wafer and particularly relates to a method for evaluation of the extent of the transition region in a silicon epitaxial wafer based on an interferogram of infrared radiation reflected from a free surface of a vapor phase growth layer and the growth interface between said vapor phase growth layer as well as the vicinity of the growth interface and the substrate in silicon epitaxial wafer. This is effected by measurement of the intensity of the interference fringes employing a Michelson interferometer in which infrared radiation is used as a light source for the measurement.
Epitaxial growth is one of the basic techniques in the industrial field of silicon semiconductor electronic device fabrication. This technique is employed not only in the manufacture of bipolar transistors and bipolar integrated circuits but also in the manufacture of MOS integrated circuits.
In the epitaxial growth technique, an epitaxial growth layer is formed on a silicon single crystal substrate doped with an active impurity, usually of high concentration, in a high temperature, depending on circumstances, in an atmosphere of gas which is apt to deposit silicon atoms through chemical reactions on the silicon single crystal. In the vicinity of the growth interface between this epitaxial growth layer and the silicon single crystal substrate, a part of an active impurity in said single crystal substrate is taken in said growth layer, whereby the region in which concentration of active impurity changes (hereinafter, referred to as "the transition region") is formed in the growth layer. This transition region consists of the thermal diffusion region which is generated by thermal diffusion of an active impurity contained in the single crystal substrate, and the auto doping region which is generated through the incorporation of the active impurity in a gaseous state during the course of chemical reaction and epitaxial growth, wherein generally both phenomena occur concurrently.
Recently, demand has been built up to form epitaxial growth layers which are considerably thinner because inter alia, to improve the high frequency characteristic of semiconductor electronic devices. At present, it is possible to form a vapor phase growth layer with a thickness under 0.5 .mu.m.
There is, however, a spreading of the transition region in the vicinity of the growth interface between an epitaxial growth layer and a single crystal substrate and a change in dopant level with regard to thickness at the transition region which have effects upon the functions of a silicon semiconductor device. Such effects upon the functions of a silicon semiconductor device by the doping of transition region cannot be disregarded even when the thickness of epitaxial growth layer is thicker, for example 10 .mu.m. Therefore, the above effects are problems which have to be solved in designing these silicon semiconductor devices.
Heretofore, the transition region in the vicinity of the growth interface between an epitaxial growth layer and a single crystal substrate is determined by various known methods, and trials are made at dimensional evaluation of the transition region. However, these methods of evaluation are complicated, and inefficient. Further, these conventional evaluation methods are destructive and practically insufficient.
The transition region can be determined, for example, by the spreading resistance method, which is conventional.
As shown in FIG. 1(a), a test piece used in the spreading resistance method is a chip 100 with the square surfaces of about 5 mm.times.5 mm, which is cut from a wafer having epitaxial growth layer e on a substrate b and which has a polished surface to be measured which is obliquely cut at an angle of 1-5 degrees to the surface. A set of osmium probes 101, 101 is moved by microscopic intervals in parallel with the upper edge in a slanting direction across the slanting surface to be measured; spreading resistances between osmium probes are then measured. From plotting the values of the measured spreading resistances with respect to the depth from the surface of epitaxial growth layer as shown FIG. 1(b), the transition region T can be obtained.
This method takes over 30 minutes time and besides is a destructive test.
Further, and conventionally, the transition region is determined by a C-V method. As shown in FIG. 2(a), in the C-V method, the capacity-voltage characteristic between a metallic electrode formed through oxide film on a surface of epitaxial wafer to be measured and an ohmic contact electrode formed on the rear of said epitaxial wafer is measured, and from these measured data, a profile of dopant concentration is obtained as a relationship of impurity concentration at a depth from the surface of the wafer as shown in FIG. 2(b). From this profile, the transition region can be obtained.
This method also takes time over 30 minutes, and again is a destructive test. Further, when the concentration of impurities is more than 5.times.10.sup.18 atoms/cm.sup.3, the spreading of the depletion layer does not occur regardless of any applied voltage so that measurement becomes impossible.